Cuong C. Do

Statistical moments-based OSNR monitoring for coherent optical systems

We present a novel statistical moments-based method for optical signal-to-noise ratio (OSNR) monitoring in polarization-multiplexed (pol-mux) coherent optical systems. This technique only requires the knowledge of the envelope of the equalized signal before phase correction, which can be achieved by using any two arbitrary statistical moments, and it is suitable for both constant and non-constant modulus modulation formats. The proposed estimation method is experimentally demonstrated for 10-Gbaud pol-mux coherent systems using QPSK and 16-QAM. Additionally, numerical simulations are carried out to demonstrate 20-Gbaud systems using 16-QAM and 64-QAM. The results show that the OSNR can be estimated accurately over a wide range of values for QPSK, 16-QAM or 64-QAM systems up to 1920-km long and with up to 50-ps all-order polarization mode dispersion. By setting a proper reference value for calibration, the proposed algorithm also shows good tolerance when the received signal is not well compensated.

Data-Aided Chromatic Dispersion Estimation for Polarization Multiplexed Optical Systems

We propose a full transmission link chromatic dispersion (CD) estimation technique for polarization-multiplexed coherent optical single carrier system using data-aided channel estimation and least square quadratic fit. New training sequences based on Chu sequence and Golay complementary pair are designed and compared for estimation performance against the ideal full-level Chu sequence for both QPSK and 16-QAM systems. The proposed technique is demonstrated in a 40-Gb/s coherent polarization multiplexed optical system over 2080-km fiber transmission for QPSK format and 80-Gb/s system over 1800-km fiber for 16-QAM format to verify its accurate and robust estimation performance.

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Owing to its low transmitter complexity and low sensitivity to nonlinear impairments, phase noise, and frequency offset, the single carrier system with frequency domain equalization (SC-FDE) is of interest for high-bit-rate optical transmission. In this letter, we experimentally demonstrate a 8×40-Gb/s optical coherent polarization-multiplexed SC-FDE over 2080-km fiber transmission using novel training sequences. The sequences are designed based on Chu and Golay complementary sequences to have binary levels and are fully compatible with the commercial quadrature phase-shift keying (QPSK) systems. The use of these training sequences helps achieve accurate channel estimation and equalization in full-system demonstration.

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We experimentally demonstrate full-transmission chromatic dispersion estimation with high accuracy in 40-Gb/s coherent polarization-multiplexed single-carrier system using binary complementary Golay training sequences. The technique is verified to be robust against polarization-mode-dispersion and amplified-spontaneous-emission noise.

40-Gb/s Optical Coherent Pol-Mux Single Carrier System With Frequency Domain Equalization and Training Sequences

We propose an optical signal-to-noise ratio (OSNR) estimation technique using self-equalized complementary Golay sequences for both QPSK and 16-QAM coherent optical systems. The training sequences have complementary spectra, which enable accurate channel estimation and equalization, and have length-independent constellation for high-performance estimation. Estimation performances are verified through 100- and 40-Gb/s QPSK and 200- and 80-Gb/s 16-QAM coherent polarization-multiplexed optical systems, with an accuracy of 0.4 and 0.7 dB, in both simulations and experiments, respectively.

Chromatic dispersion estimation in 40 Gb/s coherent polarization-multiplexed single carrier system using complementary Golay sequences

In this paper, we present a chromatic dispersion (CD) estimation technique based on complementary Golay sequences for systems employing single-carrier with frequency domain equalization (SC-FDE) format. By exploiting the unique correlation property of Golay sequences and using the sequences as channel estimation symbol (CES), chromatic dispersion can be estimated by measuring partial phase response and compensated using frequency domain equalization (FDE) with relatively low complexity in both the training sequences and estimation algorithm. The technique is verified by computer simulation to be able to estimate and compensate for up to 1500 km fiber with standard dispersion at high accuracy in 80 Gb/s coherent NRZ-QPSK system.

Data-Aided OSNR Estimation for QPSK and 16-QAM Coherent Optical System

In this paper, we present the design of digital signal processing (DSP) algorithms for training-aided coherent optical single-carrier frequency-domain equalization (SC-FDE) systems. Based on two training-aided channel estimation (TA-CE) schemes, the requirements for training sequence to achieve the minimum mean square error performance of channel estimation (CE) are outlined. Moreover, the practical implementation issues of constant-amplitude zero-autocorrelation sequence and Golay sequences are discussed. We propose to perform time-domain windowing to the CE taps for further noise suppression to improve the CE and achieve large overhead saving with the optimized CE. Furthermore, frame timing synchronization and frequency offset compensation algorithms based on Golay sequences are developed, which provide bandwidth-efficient solution with robust performance for long-haul transmission. Finally, a low-complexity fractionally spaced frequency-domain equalizer is reported to effectively reduce the computational complexity of the whole system. A total of 28-Gbaud coherent polarization division multiplexing (PDM) system simulations and 10-Gbaud coherent PDM system experiments are conducted to verify that the proposed DSP solutions provide robust performance and are suitable for implementation in high-speed long-haul digital coherent receivers.

Chromatic dispersion estimation based on complementary Golay sequences for 80 Gb/s QPSK single-carrier system with frequency domain equalization

We propose a technique for polarization-mode-dispersion (PMD) estimation in 16-QAM coherent polarization-multiplexed single-carrier system using complementary Golay training sequences and channel response filter analysis. The technique is experimentally proved to be fast and highly accurate.

Digital Signal Processing for Training-Aided Coherent Optical Single-Carrier Frequency-Domain Equalization Systems

We experimentally demonstrate coherent polarization-multiplexed QPSK and 16QAM single-carrier system with frequency-domain-equalization over 800-km in-line transmission using binary training sequences. The sequences are designed to be practical and achieve accurate channel estimation in full-system demonstration.

PMD monitoring in 16-QAM coherent optical system using golay sequences

A Stokes vector based non-data-aided (NDA) differential group delay (DGD) estimation scheme with flexible bandwidth requirements is investigated. The scheme is demonstrated in both experiments and simulations for 40-Gb/s polarization-multiplexed coherent optical OFDM systems with up to 1000-km long haul transmission with an estimation error of less than 2.5 ps after 1000-km standard single-mode fiber transmissions.